1. Field of the Invention
The present invention relates to a permanent magnet type rotating electric machine, and particularly, to a permanent magnet type rotating electric machine having permanent magnets embedded in a rotor.
2. Description of the Background Art
In a rotor of a permanent magnet type rotating electric machine, for example as shown in FIG. 10, a plurality of openings 120 are provided to a rotor core 100, and permanent magnets 130 are inserted into openings 120 to create magnetic poles (for example, see Japanese Patent Laying-Open Nos. 2004-104962, 09-215236, 2002-034187, 08-237893, 2001-286109, and 2004-254466). Then, the rotor is driven to rotate based on a rotating magnetic field created by bringing a coil on a not-shown stator side arranged to surround rotor core 100 into conduction. Since a magnetic field is created by permanent magnets 130 for each prescribed magnetic pole, a high output is achieved with a small configuration.
In the permanent magnet type rotating electric machine thus configured, generally the permanent magnets are fixed through an adhesive infilling each opening 120 of rotor core 100. However, with such a fixing method of permanent magnets, when fill properties of the adhesive relative to openings 120 of rotor core 100 is poor, permanent magnets 130 may wobble due to centrifugal force acting on permanent magnets 130 when the rotating electric machine is started or stopped, or when the load abruptly changes. This wobbling may lead to a damage of permanent magnets 130. Rotor core 100 may also be damaged due to stress concentration at the portion where the adhesive is placed.
Further, for filling openings 120 with the adhesive, complicated administration is necessary as to the temperature of the adhesive and rotor core 100, the dimension of a gap between them, and a work posture, and therefore there has been a problem of poor workability.
Accordingly, in recent years, many rotors have been disclosed wherein each permanent magnet is fixed by injecting with pressure a resin member having higher fill properties into the gap between the permanent magnet and the rotor core (for example, see Japanese Patent Laying-Open Nos. 09-215236 and 2002-034187). For example, Japanese Patent Laying-Open No. 09-215236 discloses a magnet embedded type brushless DC motor wherein a gap between a permanent magnet and a rotor core is filled with pressure with a molten die-cast filler, which is then cured, to fix the permanent magnet.
Specifically, in the rotor core, while setting the circumferential dimension of each opening to be greater than the circumferential dimension of each permanent magnet, gaps extending in an axial direction are respectively formed on opposite sides in circumferential direction of the permanent magnet. The gaps are filled with pressure with a molten die-cast filler, which is then cured. This alloy material serves to fix the permanent magnet within the insert portion.
With this configuration, by setting the dimension of the opening in the circumferential direction to be greater than the width dimension of the permanent magnet, a change in the width dimension of the permanent magnet can be addressed. Additionally, by pouring and allowing to cure the molten die-cast filler into the gaps formed on opposite sides in the circumferential direction of the permanent magnet, the permanent magnet can be fixed within the opening irrespective of a change in the width dimension of the permanent magnet and variations of the dimensional tolerance.
However, with the fixing method of permanent magnets using a filler as described above, stress may generated at the rotor core due to a difference in coefficient of linear expansion among the permanent magnets, the heated and cured filler and the rotor core.
That is, since the permanent magnets, the heated and cured filler and the rotor core are made of materials different from one another, they are different in coefficient of linear expansion. Accordingly, in a manufacturing process of the rotor, when heat is applied to cure the filler, with which the gap between the permanent magnet and the opening have been filled with pressure, each of them expands by a different amount. When the filler is cured, sometimes stress is generated at the inner circumferential surface of the opening in the direction compressed by the filler.
Additionally, such stress may be generated at the rotor core also when the permanent magnet, the filler and the rotor core expand due to heat being generated in association with the operation of the rotating electric machine.
Accordingly, receiving such stress attributed to the filler, the rotor core may greatly deform. Then, when the rotor core deforms, the length of the air gap between the outer circumferential surface of the rotor core and the inner circumferential surface of stator 5 becomes irregular, which may impair the output performance of the rotating electric machine.
Additionally, in the rotor core, as shown in FIG. 10, stress may remain at a bridge portion 140 formed between adjacent openings 120. Therefore, the reliability of the rotating electric machine is impaired.
Specifically, the bridge portion is designed to have the smallest possible width in order to prevent performance deterioration due to a magnetic flux of the permanent magnet leaking from N pole through the portion to S pole. However, the strength of the bridge portion cannot be maintained due to the aforementioned stress remaining therein, which impairs the reliability of the rotating electric machine.